Digital Multimeter - Method of measuring DC voltage and DC current
When reading, you should refer to the scale with the symbol "DC" or "V · MA". At this time, the value pointed by the knob is the maximum limit (range) of the DC voltage or current. Before measurement, the size of the measured value should be estimated first, and then the conversion switch should be turned to the appropriate limit gear. The size of the measured value can be read proportionally. When measuring the voltage between two points on a circuit, the red test pen should be connected to the high potential point, and the black test pen should be connected to the low potential point. When measuring DC current, the ammeter must be connected in series to the branch, so the measured branch must be disconnected first.
If two probes are placed on the two ends of the branch without disconnecting it, it is actually using an ammeter to measure voltage, and the ammeter will be burned out. Judging the quality of a capacitor with a multimeter depends on the capacity of the electrolytic capacitor. Usually, the r × 10, r × 100, and r × 1k ranges of the multimeter are selected for testing and judgment. Connect the red and black probes to the negative terminal of the capacitor respectively (discharge the capacitor before each test), and judge the quality of the capacitor by the deflection of the probe.
If the pointer quickly swings to the right and then slowly returns to its original position to the left, generally speaking, a capacitor is good. If the pointer no longer rotates after swinging, it indicates that the capacitor has broken down. If the pointer gradually returns to a certain position after swinging up, it indicates that the capacitor has leaked electricity. If the pointer cannot be raised, it indicates that the capacitor electrolyte has dried up and lost its capacity.
It is difficult to accurately determine the quality of some capacitors with leakage using the above methods. When the withstand voltage value of the capacitor is greater than the voltage value of the battery in the multimeter, according to the characteristics of small leakage current during forward charging and large leakage current during reverse charging of the electrolytic capacitor, the r × 10k gear can be used to reverse charge the capacitor, observe whether the pointer stays stable (i.e. whether the reverse leakage current is constant), and judge the quality of the capacitor with high accuracy. Connect the black probe to the negative terminal of the capacitor and the red probe to the positive terminal of the capacitor. If the probe quickly swings up and then gradually retreats to a certain position and stops moving, it indicates that the capacitor is good. Any capacitor whose probe stops unstable at a certain position or gradually moves to the right after stopping has leaked electricity and cannot be used anymore. The pointer generally stays and stabilizes within the 50-200k scale range.
